Enable vectorization on arm when compiled with -mfpu=neon

pocketfft_hdronly.h

@@ -140,6 +140,9 @@ template<> struct VLEN<double> { static constexpr size_t val=2; };
 #elif (defined(__VSX__))
 template<> struct VLEN<float> { static constexpr size_t val=4; };
 template<> struct VLEN<double> { static constexpr size_t val=2; };
+#elif (defined(__ARM_NEON__))
+template<> struct VLEN<float> { static constexpr size_t val=4; };
+template<> struct VLEN<double> { static constexpr size_t val=2; };
 #else
 #define POCKETFFT_NO_VECTORS
 #endif